The present application is the national stage filing of and claims priority to International Application No. PCT/EP97/04088, filed Jul. 28, 1997 and Italian Application Serial No. RM97A000146, filed Mar. 14, 1997.
The present invention refers to a process for the inhibition control in the production of grain-oriented electrical sheets and, more precisely, it refers to a process by which, through control of copper, aluminium and carbon content, type and quantities of precipitated second phases are determined since the hot-rolled strip, to obtain optimum grain size during the decarburization annealing and some degree of inhibition, thus allowing to carry out a subsequent continuous high-temperature heat treatment in which aluminium as nitride is directly precipitated by diffusing nitrogen along the strip thickness, in order to obtain the second phases ratio necessary to control the grain orientation of the final product.
Grain-oriented silicon steels for magnetic uses are normally classified into two groups, essentially differentiated by the induction value induced by a magnetic field of 800 As/m and known as xe2x80x9cB800xe2x80x9d: the conventional grain-oriented group, where B800 is lower than 1890 mT, and the high-permeability grain-oriented group, where B800 is higher than 1900 mT. Further subdivisions are depending on the so-called xe2x80x9ccore-lossesxe2x80x9d, expressed as W/kg.
Conventional grain-oriented steel, used since the 1930""s, and grain super-oriented steel, having a higher permeability and industrially used since the second half of the 1960""s, are essentially used to realise cores for electric transformers, the advantages of the super-oriented steel rising from its higher permeability (which allows reductions of core sizes) and from its lower losses, which are energy-saving.
The permeability of the sheets depends on the orientation of the body-centred cubic-lattice iron crystals (or grains): one of the grain edges must be parallel to the rolling direction. By using some precipitates (inhibitors), also called xe2x80x9csecond phasesxe2x80x9d, of suitable sizes and distribution, which reduce grain boundary mobility, a selective growth of the sole grains having the wanted orientation is obtained; the higher the dissolution temperature of said precipitates into the steel, the higher the grain orientation and the better the magnetic characteristics of the final product. Manganese sulphide and/or selenide are the predominant inhibitors in an oriented grain steel, while precipitates including nitrogen linked to aluminium (referred to as xe2x80x9caluminium nitridexe2x80x9d for simplicity purposes) are the predominant inhibitors in a super-oriented grain steel.
Nevertheless, when a grain-oriented sheet or a grain super-oriented sheet is produced, during the solidification of steel and the cooling of the solidified body, second phases allowing the above mentioned improving effect are precipitated in coarse form, useless for the wanted purposes; said second phases must be therefore dissolved, reprecipitated in the right form and maintained into said form until the grain having wanted sizes and orientation is obtained at the end of a complicated and expensive transformation process including a cold-rolling at the desired final thickness, a decarburization annealing and a final annealing.
It is evident that the production problems, linked essentially to the difficulties of obtaining high yields and constant quality, are mainly due to the precautions to be taken during the whole transformation process of the steel for maintaining the second phases and, in particular, the aluminium nitride in the wanted form and distribution. In order to relieve said problems, technics have been developed where, the aluminium nitride suitable to control the grain growth is obtained by nitriding the strip, preferably after cold-rolling, as it is disclosed in U.S. Pat. No. 4,225,366 and in European patent n. 0,339,474.
According to the last mentioned patent the aluminium nitride, coarsely precipitated during the slow steel solidification, is maintained into said state by using low slab-heating temperatures (lower than 1280xc2x0 C., preferably lower than 1250xc2x0 C.) before the hot-rolling; nitrogen is introduced after the decarburization annealing, which reacts immediately to produce, essentially near the strip surfaces, silicon and manganese/silicon nitrides, having comparatively low solution temperature, which are dissolved during the final annealing in box-annealing furnaces; the nitrogen so released diffuses into the sheet, reacts with the aluminium and precipitates again on the whole strip thickness in a thin and homogeneous form as mixed aluminium and silicon nitrides; said process involves that the material stays at 700-800xc2x0 C. for at least four hours. The above patent states that nitrogen must be introduced at a temperature near the decarburization one (about 850xc2x0 C.) and in any case no higher than 900xc2x0 C. to avoid an uncontrolled grain growth due to the absence of suitable inhibitors. In fact, the optimum nitriding temperature should be of about 750xc2x0 C., while 850xc2x0 C. is the upper limit to avoid said uncontrolled growth.
Prima facie the above process has some advantages: relatively low slab-heating temperatures before hot-rolling, decarburation and nitriding, and the fact that no increase in production costs is due to the necessity to maintain the strip at 700-850xc2x0 C. for at least four hours in the box-annealing furnace (to obtain the mix of aluminium and silicon nitrides required to control the grain growth), as the heating in the box-annealing furnaces in any case requires similar times.
However, together with the above mentioned advantages the above process has some disadvantages as: (i) due to the low slab-heating temperature the sheet includes practically no precipitates inhibiting the grain growth: all the heating steps of the strip, and in particular those belonging to the decarburization and to the nitriding steps, must be taken at comparatively low and critically controlled temperatures, in that at the above conditions grain boundaries are very mobile involving the risk of an uncontrolled grain growth; (ii) the nitrogen introduced is stopped near the strip surfaces as silicon and manganese/silicon nitrides, which must be dissolved to allow the nitrogen diffusion towards the core of the sheet and its reaction for creating the wanted aluminium nitride: as a consequence, no improvement speeding up the heating time can be introduced during the final annealing, for example by using another type of continuous furnace instead of box-annealing ones.
The Applicant, knowing the above difficulties, has developed an improved process which is new and involves a considerable inventive step over the prior art, from which it is distinguished with regard to both the theoretical bases and the process characteristics.
Such process is disclosed by Applicant""s copending U.S. applications Ser. Nos. 09/243,000, filed Feb. 26, 1999; 09/242,992, filed Feb. 26, 1999; 09/331,273, filed Jun. 17, 1999; 09/331,506, filed Jun. 22, 1999; and 09/331,504, filed Jun. 22, 1999.
Said patent Applications clearly set forth that the whole process, and in particular the control of the heating temperatures, can be made less critical if some precipitation of inhibitors suitable to control the grain growth is allowed since the hot-rolling step, thus permitting a best control of the grain size during the primary recrystallisation (during the decarburization annealing) and then a deep nitriding of the sheet to directly create aluminium nitride.
In more detail, the process described in said patent applications provides for carrying out said primary recrystallization annealing continuously between 800 and 950xc2x0 C., in a wet nitrogen-hydrogen atmosphere for a period of time of between 20 and 150 seconds to produce a primary recrystallized strip; continuously nitriding said recrystallized strip at a temperature between 850 and 1050xc2x0 C. [for a time between 5 and 120 seconds], in a wet nitriding atmosphere comprising ammonia at a level of from 1 to 35 standard liters per kg. of strip and from 0.5 to 100 g/m3 of water vapor.
Object of the present invention is to overcome the disadvantages of the production processes already known and to further improve the technology disclosed by the above mentioned Italian patent Applications by disclosing a process for creating and for controlling, since the hot-rolling step, a system of various inhibitors suitable to make less critical most of the production steps, with particular reference to the careful control of the heating temperature, to obtain optimum grain sizes during the primary recrystallisation and a deep penetration of the nitrogen into the strip to directly form aluminium nitride.
According to the present invention, through a suitable combination of contents of carbon, aluminium and copper it is possible to make easier, according to the innovative technology disclosed by the above mentioned Applicant""s Italian patent Applications, the production of silicon steel sheets both of grain-oriented type and of grain super-oriented type.
In particular, according to the invention, the control of the content of copper, carbon and aluminum within the ranges of 800-1800 ppm, 50-550 ppm, 250-350 ppm respectively allows to obtain since the hot-rolled strip thin precipitates and in particular precipitates including nitrogen linked to aluminum and mixed copper+manganese sulfides, apt to give to the sheet an effective inhibition (Iz), between 400 and about 1300 cmxe2x88x921 suitable to control the grain dimensions of the decarburized product.
The effective inhibition is calculated through the empirical formula:
Iz=1.91 Fv/r
where Fv is the volume fraction of useful precipitates and r is the mean radius of said precipitates.
Preferably, the copper content is controlled in the 1000-1500 ppm range. The carbon content is preferably in the 50-250 ppm range for a final oriented grain, while it is comprised in the 250-550 ppm range for a final super-oriented grain.
The aluminium content is preferably controlled in the 280-310 ppm range.
Still according to the invention, the continuously cast slabs are heated between 1150xc2x0 C. and 1320xc2x0 C., preferably between 1200xc2x0 C. and 1300xc2x0 C., and hot-rolled.
Then, the hot-rolled strip is quickly heated to 1100-1150xc2x0 C., cooled to 850-950xc2x0 C. left at such temperature for 30-100 s and then quenched from 550-850xc2x0 C.
The cold-rolling preferably includes passes performed at a temperature comprised between 180 and 250xc2x0 C.
The final decarburization and nitriding treatments can be performed in various alternative ways, such as:
(i) in a single step, wherein the decarburization is carried out in a wet nitrogen-hydrogen athmosfere, ammonia being added in the final part of the process;
(ii) in a double step, wherein ammonia is added only after having completed the decarburization process, preferably by rising the treatment temperature up to a maximum of 1050xc2x0 C.;
(iii) in a double step, wherein ammonia is added both after having completed the decarburization process and afterwards, always in the continuous furnace; also in this case it is preferred to rise the treatment temperature up to 1100xc2x0 C. at the final nitriding stage.
The strip coated with MgO-based annealing separators and coiled, is box-annealed by heating it up to 1210xc2x0 C. under a nitrogen-hydrogen athmosphere and keeping it for at least 10 h under hydrogen.
The present invention will be now disclosed through some embodiments.